Everything You Need to Know About Vitamin D

It sounds like science fiction that simple exposure to sunlight would be enough for our body to create one of the essential nutrients we need for good health. But it’s true–sunlight exposure leads to vitamin D synthesis in the body, and that’s important for the health of our bones, skin, and teeth–and potentially for much more.

The National Health and Nutrition Examination Survey done in 2005 and 2006 found that while 37 percent of the population of the United States used a dietary supplement with vitamin D, adults across the board were taking in less than the RDA for the nutrient. And over the past 20 years, mean serum rates of vitamin D, measured by blood tests, have declined overall in men but not women–probably related to increased body weight, reduced milk intake, and greater use of sunscreen.

Here is an overview of what you need to know about vitamin D, from how it works in your body to where it shows up in your diet and why you might consider supplements.

Vitamin D in the Body

Vitamin D is a fat soluble vitamin required for the proper functioning of our bodies. For starters, the vitamin promotes the proper absorption of calcium, according to the National Institutes of Health, which means that adequate vitamin D levels are important for the proper use of other nutrients. The vitamin is related to bone growth and strength, and sufficient levels of vitamin D in our bodies prevents rickets in children and osteomalacia in adults. Vitamin D also plays a role in osteoporosis prevention–insufficient vitamin D intake can reduce the absorption of calcium, which is a risk factor for the condition.

In addition to those important functions, vitamin D is also related to cell growth, neuromuscular and immune function, and inflammation control, according to the NIH. A deficiency of vitamin D in children can lead to rickets, a disease characterized by soft bones and skeletal deformities. In adults the deficiency causes osteomalacia, which also results in bone weakness. There is preliminary research suggested that vitamin D levels might be related to cancer risk, though the information is still preliminary.

Winter and the Sunshine Vitamin

When ultraviolet rays from the sun hit our skin, vitamin D synthesis is triggered. However, the amount of vitamin D an individual synthesizes varies based on factors like season, time of day, length of day, cloud cover, smog, skin melanin content, and sunscreen application, the NIH says. If you live in a part of the world where exposure to sunlight is limited during some parts of the year–which includes Canada and a significant portion of the U.S.–then there are long periods of time where you may not be getting enough ultraviolet exposure to fill your needs for vitamin D, both because exposure to sunshine is limited during the shorter days and because we’re all wearing more clothing when the weather is colder. And if you don’t go outside often, you might also not be meeting your needs–UVB rays don’t penetrate glass, which means that sunlight exposure through windows doesn’t lead to vitamin D synthesis.

Vitamin D in Your Diet

Vitamin D is not found naturally in many foods–the best natural sources are cod liver oil, salmon, tuna, and sardines. As you can imagine, if you don’t eat fish or aren’t a big fan of certain fish–and not choking down cod liver oil regularly–then you might not be getting much vitamin D in your diet.

Some foods are supplemented with vitamin D–most notably milk, which is actually naturally low in the nutrient. Other dairy products like yogurt are also fortified with vitamin D, as are some cereals and orange juices. Supplementation of milk began in the 1930s as a campaign to reduce rickets in children, which proved effective–it’s now rare in the United States and Canada.

Because so few foods contain vitamin D, either naturally or through supplementation, many people choose to take supplements. For people with a limited diet–for example, vegetarians or people who can’t or don’t consume dairy or some grains–getting enough vitamin D can be even more difficult.

There are two forms of vitamin D available in supplements: vitamin D2 and vitamin D3. Vitamin D2 is made through the UV radiation of ergosterol in yeast, which means it’s a vegetarian source of the vitamin. Vitamin D3 results from the irradiation of 7-dehydrocholesterol from sheep lanolin. The NIH says that it appears that both forms of the vitamin are equally effective at nutritional doses, but that vitamin D2 is less potent in high doses.

The American and Canadian RDAs for vitamin D is currently 400 IU for infants, 600 IU for children and teens, as well as for adults aged up to 70 years old. For those older than 70, 800 IUs are recommended.